The invention relates to a long-term corrosion resistant storage container fabrication for isolating radioactive waste material in a high level waste repository and, more particularly, relates to such a container having a plurality of relatively light weight, thin walled, hermetically sealed canisters mounted within one another in combination with selected spacer means disposed between adjacent canisters to support them in a predetermined spaced relationship. The plurality of canisters and spacer means used to fabricate a container according to the invention are effective to greatly increase the long-term corrosion resistance of the container relative to the corrosion resistance life of other storage containers that may have equal or greater wall thicknesses and greater overall weights in their wall construction materials.
From the time that the first radioactive materials were produced, to the present time, there has existed a desire to develop storage containers in which such materials could be safely deposited until their levels of radioactivity decayed sufficiently to render them safe to handle. As the evolution of nuclear power generating stations occurred, increasingly large volumes of radioactive waste material were produced on an annual basis. Concurrently, operating practices and management guidelines were developed to provide for the safe handling, short-term storage, and long term disposition of these large volumes of so-called radwaste materials. In general, under such practices and guidelines, different procedures were developed for handling both relatively low level radioactive materials, and high level radioactive materials.
A primary codification of the guidelines that has been developed in the United States for handling such radwaste materials is set forth in the U.S. Code of Federal Regulations. Specifically, 10 CFR, Part 60 sets forth the Nuclear Regulatory Commission's criteria and guidelines for storing high level radwaste materials in multiple-layer containers that must be sufficiently resistant to corrosion to prevent the containers from leaking for a minimum period of 300 years. 10 CFR, Part 61 sets forth the NRC licensing requirements and criteria for disposing of relatively low level radwaste materials in shallow sand disposal sites. Such relatively low level radwaste materials should be completely contained in high integrity waste containers that have as a design goal a life-time of not less than 300 years. The corrosive and chemical effects of both the radwaste contents of the containers and of their environment in a repository will determine their usable life span. Accordingly, suitable tests must be devised for ascertaining the corrosion resistance and chemical characteristics of proposed materials and designs that are to be used in making such containers.
The present invention is useful in manufacturing corrosion resistant storage containers for safely isolating high level radwaste material in compliance with the requirements of 10 CFR 60, i.e., for making containers that resist corrosion sufficiently to prevent leakage from the containers for 300 years. Because the repositories in which such containers may be stored might include environments that contain water or other corrosion facilitating liquids, the container constructions of the invention are selected to maximize the corrosion slowing or inhibiting effect by requiring successive initiation of corrosion on sequentially spaced surfaces of the canisters making up the container. In other words, because corrosion of most metals is slow to initiate on a metal surface, but then proceeds rapidly through the sub-surface portions of the metal, one principle of the present invention is to force corrosive materials to sequentially encounter a series of separate surfaces upon which corrosion must be independently initiated. A second principal of the invention is to maximize the dilution or weakening of the corrosive effect of liquids or other contaminants as they penetrate the outer canisters and spacer means of the container, so that the corrosive effect of such penetrating contaminants on the inner canisters of the container is minimized or nearly neutralized. The combined effect of these principles is that containers made in accordance with the invention have desirably non-linear rates of corrosion. The existing laws regulating the isolation and disposal of high level radioactive wastes make it necessary to extrapolate data from relatively short-term corrosion rate tests to predict corrosion rates for periods up to 1000 years with reasonable assurance of the accuracy of the predictions. To provide such reasonable assurances, the extrapolation methods should be conservative. A linear extrapolation of corrosion rates for a container would be conservative if the container can be shown to have a non-linear and much slower than linear corrosion rate. Thus, the present invention provides a solution to the long standing problem of providing a conservative means for reasonably assuring that short term corrosion rate test data can be used to predict the corrosion rates for containers over anticipated 1000 year life spans.
It is well known in the nuclear industry to provide multiple layered containers for handling radioactive material. Such prior art multiple layer radioactive material containers have generally been designed to provide two basic functions. First, containers having multiple layers of thick metal, concrete or other radiation shielding means are frequently used to protect those handling the containers from the lethal effect of the radiation emitted by the material within the containers. Second, rugged, relatively heavy multiple-layered containers have been so constructed in order to resist the mechanical shocks encountered in shipping radioactive materials. Containers designed for providing those well known, common functions are readily distinguished from the type of thin-walled, light weight plural canister and spacer means construction used in fabricating containers according to the present invention. For example, the heavy shock resistant type of containers used for transporting radioactive material normally contain a number of unsealed joints in their sidewalls which enable the containers to be readily opened for insertion and removal of the shipped contents. In addition to containing such leakage-prone or weakly corrosion-resistant joints, which are typically of a simple step design, such shipping containers normally utilize different materials in the respective multiple layers thereof, so that should corrosion be initiated through the walls of the container an undesirable electrolytic action would readily be established to accelerate the progress of such corrosion through successive layers of the containers. Examples of such prior art multiple layer containers for transporting radioactive materials are shown in U.S. Pat. Nos. 3,575,601--Graham, which issued Apr. 20, 1971; 3,780,306--Anderson, which issued Dec. 18, 1973; 3,845,315--Blum, which issued Oct. 29, 1974; and 3,930,166--Bochard, which issued Dec. 30, 1975.
Examples of the type of prior art multiple layer containers that have been designed primarily for shielding the environment from radiation emitted by the contents of the containers are shown by U.S. Pat. Nos. 3,780,309--Bochard, which issued Dec. 18, 1973; 4,006,362--Mollon, which issued Feb. 1, 1977; and 4,058,479--White, which issued Nov. 15, 1977. The types of containers represented by the last three designated patents utilize successive layers of different materials to increase their radiation shielding effect; however, as pointed out above, it should be recognized that the use of such different materials provides an undesirable corrosion enhancing mechanism due to the establishment of electrolytic cells between the different materials of the container as corrosion introduces electrolytes into contact with the successive layers in the walls of the containers. Moreover, it appears that the types of multiple-walled containers shown by these last three patents are primarily suited for short term storage of relatively low level radwaste materials, rather than being designed to be effective for long term storage of high level radwaste materials. In practice, it is now common to use either 55 gallon carbon steel drums lined with lead or concrete, or to use 30 gallon concrete containers to package low level radwaste materials for disposal in an approved land burial site.
The plural canister container of the present invention is readily distinguished from such known types of shorter-term storage containers for low level radwaste materials, and from the prior art types of multiple-layer shipping containers shown, respectively, in the foregoing patents. For example, the spacer means used in fabricating some preferred embodiments of the containers of the present invention are selected to have interstices and high surface areas that inhibit free flow of corrosion-accelerating liquids between the surfaces of successive canisters within a given container. In addition, diffusion of reactants from the exterior of corroding surfaces of the successive canisters in a container fabricated according to the invention are inhibited by such spacer means disposed between the successive canisters. A further characterizing advantage of the container construction of the present invention is that removal of beneficial corrosion products by free-flowing liquids is prevented, and the formation of local corrosive concentration cells on the surfaces of the canisters of the containers is prevented by the particulate, porous materials used in forming some of the spacer means employed in practicing certain embodiments of the invention.